Role of Ni in PtNi Bimetallic Electrocatalysts for Hydrogen and Value-Added Chemicals Coproduction via Glycerol Electrooxidation

Pt-based bimetallic electrocatalysts are promising candidates to convert surplus glycerol from the biodiesel industry to value-added chemicals and coproduce hydrogen. It is expected that the nature and content of the elements in the bimetallic catalyst can not only affect the reaction kinetics but also influence the product selectivity, providing a way to increase the yield of the desired products. Hence, in this work, we investigate the electrochemical oxidation of glycerol on a series of PtNi nanoparticles with increasing Ni content using a combination of physicochemical structural analysis, electrochemical measurements, operando spectroscopic techniques, and advanced product characterizations. With a moderate Ni content and a homogenously alloyed bimetallic Pt–Ni structure, the PtNi2 catalyst displayed the highest reaction activity among all materials studied in this work. In situ FTIR data show that PtNi2 can activate the glycerol molecule at a more negative potential (0.4 VRHE) than the other PtNi catalysts. In addition, its surface can effectively catalyze the complete C–C bond cleavage, resulting in lower CO poisoning and higher stability. Operando X-ray absorption spectroscopy and UV–vis spectroscopy suggest that glycerol adsorbs strongly onto surface Ni(OH)x sites, preventing their oxidation and activation of oxygen or hydroxyl from water. As such, we propose that the role of Ni in PtNi toward glycerol oxidation is to tailor the electronic structure of the pure Pt sites rather than a bifunctional mechanism. Our experiments provide guidance for the development of bimetallic catalysts toward highly efficient, selective, and stable glycerol oxidation reactions.

High-resolution transmission electron microscopy (HR-TEM) images were obtained by a FEI Titan 80/300 at an operating voltage of 300 kV. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images, energy-dispersive X-ray (EDX) mapping and electron energy loss spectroscopy (EELS) were carried out with a probe aberration-corrected analytical electron microscope (JEOL ARM200F) in STEM mode with electron beam voltage set at 200 kV. The X-ray diffraction (XRD) patterns were performed using PANanalytical's X'PERTPRO X-ray diffractometer with Ni filtered Cu Kα radiation and an X'Celerator multistrip detector. The scanned two theta ranges from 15 to 90 at 3 /min. To reduce the noise in the final signal, the sample holder was rotating at 90 per second. The Xray photoelectron spectroscopy (XPS) of all catalysts were conducted on a Thermo Scientific Nexsa XPS system with a monochromatic Al Ka X-ray source. C 1s spectra were used to calibrate the peaks in all samples. Pt and Ni contents in each catalyst were quantified with inductively coupled plasma mass spectrometry (ICP-MS) using Agilent Nu Instruments with Nu Plasma multiple collectors. Before analyzing, the materials of ~ 5 mg were carefully weighted and digested by 8 mL aqua regia using a MARS 6 microwave at 1200W for 15 min. the solutions were filtered and diluted with 2% HNO3 and 1% HCl matrix to low metal contents (< 500 ppb). For each sample, two duplicates were measured for accuracy with the error below 5%. The reported values for all samples are the average of the two duplicates. for Ex-situ Pt L 3 edge and Ni K edge X-ray adsorption spectroscopy (XAS) measurements were performed at XMaS (BM28) beamline in European Synchrotron Radiation Facility (ESRF) and I20-EDE beamline in diamond light source Pt and Ni foils with 25 µm thickness were used to calibrate the Si (111) double-crystal monochromator and used for data collection as a Pt standard afterwards. Additional reference compounds such as platinum (IV) oxide (PtO 2 ), nickel oxide (NiO) and nickel hydroxide (Ni(OH) 2 ) were also measured. The XAS data was processed with Athena and Artemis software, and the S 0 2 for both Pt and Ni edge samples was obtained by fitting the Pt and Ni foils.

Electrochemical Experiments
Electrochemical experiments with product analysis were carried out in 3-electrode setup in one compartment PTFE cell. 5mm diameter rotating disk electrode (RDE) was used for activity measurements, and catalyst coated carbon paper (Freudenberg H23) was used for product analysis. The catalyst inks were prepared by dispersing 4mg of catalysts in 1400 μL water, 528 μL ethanol and 71 μL 5 wt % Nafion solution, followed by probe sonication for 10 min. For RDE measurement, 10 μL ink was coated onto the glassy carbon rod on RDE. For carbon paper electrode (1 cm 2 ), the inks were spray coated onto the carbon paper to achieve 0.5 mg cm -2 catalyst loading.
The electrolyte was prepared by dissolving KOH (suprapur, Merck) in ultrapure water (18.2 MΩ), and diluting pure glycerol (99.5%). A Pt mesh counter and Hg/HgO reference electrode were employed. The reference electrode was calibrated regularly against the RHE scale by measuring the hydrogen evolution/oxidation potential at a Pt rotation disk electrode (RDE) in H 2 saturated electrolyte. N 2 gas (zero grade N4.8, BOC) was bubbled for 30 min before each measurement.
The activity and stripping experiments were carried out using RDE (5 mm diameter glassy carbon, 0.196 cm 2 ) and an Autolab potentiostat. The electrolyte is 1 M glycerol + 1 M KOH. Cyclic voltammetry for all catalysts was performed between 0.1 -1.1 V vs. RHE at scan rate of 50 mV s -1 . Chronoamperometry was performed at 0.9 V vs. RHE for 2 hours to investigate the short-term stability. Mass and specific activities were compared at the peak potentials for all catalysts.
The product analysis experiments were conducted using the 1 cm 2 catalyst coated carbon paper as the working electrode, along with a Pt mesh counter and Hg/HgO reference electrode in a 20 mL volume one compartment PTFE cell. For each measurement, 10 mL of 0.1 M glycerol + 1 M KOH electrolyte was used, and the product solution was collected after 1 hour electrolysis at constant potential of 0.5, 0.7, 0.8, 0.9 and 1.1 V vs. RHE, respectively.
CO stripping was used to determine the electrochemical surface area (ECSA) and surface catalytic characteristics. The electrode was polarized at 0.2 V vs RHE and CO(g) was bubbled in the cell for 15 minutes, then N 2 (g) was bubbled for 30 minutes, followed by a cyclic voltammetry in a range of 0.1 V -1.1 V at 50 mV.s -1 . ECSA values were evaluated from the CO oxidation charge of the linear background corrected first stripping cycle under nitrogen atmosphere. This measured charge Q CO was then normalized using the theoretical value of Q CO theo =420 μC cm −2 for a two-electron transfer assuming the oxidation of one CO to CO 2 per Pt atom. 2 Glycerol stripping measurements were performed by bringing the working electrode into contact with 0.1 M glycerol + 1 M KOH electrolyte and keeping the potential at 0.2 V vs. RHE for 5 mins, followed by flushing out the electrolyte with excessive amount of 1 M KOH for 30 min. 8 CVs are then performed to ensure all the glycerol adsorbed on the catalyst's surface has been oxidized due to the slow kinetics. 3

Operando XAS
The operando XAS measurements were conducted at the i20-EDE beamline in diamond light source, using a self-designed electrochemical cell made from PEEK. Before measuring the materials under electrochemical condition, XAS spectra were recorded under dry N 2 , and later used as the reference. The potential was controlled with a Ivium OctoStat200 potentiostat and applied between a platinum wire (i.e. the counter electrode) and the sample (i.e. the working electrode). The potential at the working electrode was measured with respect to an Ag/AgCl reference electrode saturated with KCl, which was calibrated against RHE prior coming to the beamline. During the measurement, N 2 -purged electrolytes were circulated through the cell to prevent air contamination. The electrode potential was incrementally increase. All the potentials were iR corrected. Each potential was held for 15 mins during the X-ray data acquisition.

Optical-electrochemistry
For optical-electrochemistry measurements, samples were deposited on ~1cm x 1cm area of FTO substrates. Measurements were made in a three-electrode cell using a home-built optical spectroscopy setup. A stabilized 10mW tungsten-halogen light source from Thorlabs (SLS201L) was used with a collimating add on (SLS201C). The light emitted from the lamp was transmitted through the sample and collected using a 1 cm diameter liquid light guide (Edmund optics). Light transmitted to the spectrograph was first collimated and refocused using two 5 cm planoconvex lenses (Edmund) in order to optimally match the optical components of the spectroscope (Kymera 193i, Andor), CCD camera (iDus Du420A-BEX2-DD, Andor). The detector was maintained at -80 0 C during the measurements to ensure high signal-to-noise ratio. An Ivium Vertex potentiostat was used. Data acquisition was facilitated by a custom-built LabView software. Measurements were made in potentiostatic mode. The equilibration time at each potential was 10 seconds. This was followed by measurement of the optical spectra. At each potential, 30 averages of the spectra were taken (each spectral acquisition takes ~30 ms), before moving to the next potential. Simultaneously, the current was measured at each potential using the Ivium Vertex potentiostat. The potential was measured with respected to a Ag/AgCl reference electrode (saturated KCl), which was calibrated versus the reversible hydrogen electrode. A Pt mesh was used as the counter electrode.

Product Analysis
High-performance liquid chromatography (HPLC) was performed with an Agilent 1260 Infinity II LC System to detect the liquid products produced during the electrochemical oxidation of glycerol. Different compounds were separated with an Aminex HPX-87H column kept at 65 °C and detected by a refractory index detector at 50 °C. The mobile phase was 5 mM H 2 SO 4 with a flow rate of 0.4 mL min -1 , and the injection volume was 1 μL. 160 μL reaction products were collected after 10 min chronoamperometry continuously for the online sampling experiments. For faradaic efficiency and partial current density determination, 200 μL reaction products were analyzed after 1 hour chronoamperometry at 0.5, 0.7, 0.8, 0.9 and 1.1 V RHE , respectively.

In situ FTIR
In situ Fourier Transform Infrared (FTIR) experiments were performed using a Shimadzu IR-Prestige-21 spectrometer with a mercury cadmium telluride (MCT) detector. A spectroelectrochemical cell equipped with a CaF 2 prism placed on top of a specular reflection accessory (Veemax II, Pike Technologies) was used for the measurements. More details can be found elsewhere. 4 The preparation of the working electrode (WE) consists of depositing 60 µL of the nanoparticle ink on the surface of the vitreous carbon electrode. Prior to each experiment, in a three electrodes electrochemical cell containing 1 M KOH, a CO stripping were performed.
After the preparation of the electrode, it was polarized at 0.1 V, and then inserted in the spectroelectrochemical cell containing 0.1 M glycerol + 1 M KOH, put in contact with the electrolyte and pressed onto the prism to form the thin layer. The spectra were obtained during a stepped chronoamperometry in a range of 0.1 V -1 V. At each potential, 256 interferograms with a resolution of 4 cm -1 were acquired. The spectra were plotted as the ratio R/R 0 , where R 0 is the reference spectra obtained at 0.1 V and R is the spectra in function of potential. All measurements were performed in D 2 O and all potentials were applied vs RHE.

× 100%
Where and are the initial and final moles of glycerol, , and are the moles of C 1 , C 2 and C 3 products, respectively.
While Faradaic efficiency (FE%) is determined based on: 6 Where F is the Faradaic constant (96,485 C mol -1 ) and n is the electron transfer number.    Table S1.